The present invention relates to a cleaning composition for electrocleaning, and particularly to a cleaning composition for electrolytically cleaning cold-rolled steel with high current density.
If it is intended that a thinner steel coil be manufactured, the steel work can be subjected to the treatment of cold-rolling. In doing this, a roll coolant must be added for dissipating the heat generated by mechanical rolling on the surface of the steel work. Therefore, subsequent to the cold rolling operation, roll coolant which primarily includes animal oil or mineral oil together with other soils, such as iron smut, will be left on the surface of the cold-rolled steel. Before the cold-rolled steel work is subjected to annealing treatment, such soils should be thoroughly removed from the surface. Otherwise the residual oil smudge will be cracked into carbon residue or a lower carbon compound which is deleterious to the quality of the surface of the resultant steel plate, and this problem manifests itself in a poor finishing job, e.g. poor adherence in electroplating.
Due to its property of saponifying fats and oils to make water-soluble soaps, its capabilities of attacking organics and splitting esters, sodium or potassium hydroxide has been used as the most important alkali for metal cleaning. Particularly, its highest conductivity renders it an indispensible component in electrolytic cleaning composition.
It has been also described that when compounded with surfactants, silicates are the best emulsifying and deflocculating agents of all the alkali. Also their excellent buffer function in high basicity make them necessary for long-life electrocleaning compositions. On the other hand, silicates can be a possible source of trouble in subsequent plating operations and thus are suggested not to be included in the cleaning compositions for some metal cleaning processes. Sodium orthosilicate has been reported as one silicate which is widely used in steel cleaners.
Chelating agents have acquired an important role in conventional cleaning formulations in the case that little or no phosphate should be included. The most widely used chelating agents in metal cleaners are sodium gluconate, trisodium nitrilotriacetate and EDTA. These compounds can soften water and tie up many metal ions so as to enhance the cleaning effect of the cleaners.
Evidently, decreasing surface and interfacial tension will help in washing out the oil from the surface. However, specific selection of surfactants is important in electrolytic cleaning. Though nonionic surfactants have been used in combination with anionics in soak and spray cleaners, they have not been positively disclosed or suggested to be used in electrocleaners. Only the anionic type of surfactants has been disclosed for the purpose of electrocleaning. Finally, it is particularly desired in high-current-density electrocleaning to select a combination of surfactants which have good defoaming properties, because in electrocleaning with high current density, an enormous amount of hydrogen and oxygen evolves giving rise to voluminous foams which adversely cause current loss and affect the efficiency of the electrolysis.
It is known that hexamethylenetetramine can be used as a pickling inhibitor in hydrochloric and sulfuric acid. However, to the knowledge of the inventors, no literature has disclosed hexamethylenetetramine used in a formulation for electrolytic cleaning.
In view of the fact that conditions needed for conducting a electrolytic cleaning of a rolled metal sheet with high current density are very unique, to figure out a suitable cleaning formulation which can perform optimal cleaning function involves an enormous amount of experiments in light of the general teachings as discussed above.